Fire biomarker in Saharan dust traced across the Atlantic Ocean

Publication date: Monday 12 March 2018

Category: DUST

In their new paper that was published in Geochimica et Cosmochimica Acta today, Laura Schreuder and colleagues apply a new proxy for burnt vegetation; the anhydrosugar levoglucosan, and demonstrate how this sugar can be found in present-day Saharan dust.

In our studies of Saharan dust we are continuously looking for new ways to characterise the dust particles that are blown across the Atlantic Ocean. From a sedimentological viewpoint the most used tools are particle size, mineralogy and chemistry.

However, there are more ways to study wind-blown particles and one of them is by looking at organic compounds, so-called biomarkers. One of these biomarkers is the product of the burning of cellulose in vegetation: the anhydrosugar "levoglucosan". In their new study, Laura and colleagues demonstrate that indeed levoglucosan is a very valuable new proxy to quantify the amount of soot particles travelling through the air with dust.

What's more is that this sugar molecule does also preserve nicely when settling down the water column and can hence also be found in sea-floor sediments. This has major implications as it can thus potentially also be reconstructed from the sediment archive and hence applied as a proxy for fires in the past.

Here you see how so-called drifting traps are being deployed from aboard RV Pelagia by NIOZ technicians Barry Boersen (l), Jan-Dirk de Visser (r) and Martin de Vries (back). During a period of 24 hours, these traps have been drifting freely while collecting material that was sinking down from the surface ocean to the deep. This material includes dust (you can see the hazy background, which is actually Saharan dust!) but also biogenic material including remains of plankton and algae and also fire-products such as levoglucosan.

This figure shows the seasonal changes in fire activity on the African continent and how the products of these fires, including the sugar levoglucosan - a typical biomarker that is produced when cellulose burns - are blown across the Atlantic Ocean. Some of this material ended up in the sediment traps that had been recording material sinking from the surface ocean to the deep. For the first time, Laura and her colleagues demonstrated how this sugar can be found in particle sinking through the water column and ending on the sea floor.

Why are we so interested in dust?

It turns out that there are many direct and indirect links between dust and climate. The most straightforward one is fine-grained dust in the upper atmosphere blocking incoming sunlight, causing a net cooling effect. But there are warming effects too; in the lower atmosphere, coarser-grained dust particles absorb energy that was refelected at the earth's surface and this way, dust acts as a greenhouse gas.

There are many more negative and positive climate-related effects but the main link to the ocean is the fact that marine life may profit from nutrients in dust. When plankton reproduces, it takes up CO2 from the atmosphere. Thus, dust could potentially act as an ocean fertilizer, sequestering a greenhouse gas!